Continuous immuno-virological and biochemical investigations stronly suggest that the Epstein-Barr Virus (EBV), the etiological agent of mononucleosis, is one of the major contributing factors in the multifactoral etiology of nasopharyngeal carcinoma (NPC) and Burkitt's lymphoma (BL). Other contributing factors, such as chemicals and other viruses, cannot be overlooked as contributors to the complex etiology of NPC and BL, since either or both might be present to a significant degree in the environment or as part of the normal diet of certain ethnic groups. Moreover, Herpesvirus saimiri (HVS) and Herpesvirus ateles (HVA) in nonhuman primates and in rabbits appear to be etiologically associated with lymphoproliferative neoplasia. Immunological studies of nonhuman primate viruses, as well as of other EBV-like agents derived from Old World primates further support an implied association between herpesvirus infection and certain neoplasms of man. Furthermore, such nonhuman primate virus systems are ideal models for turmorigenesis testing and evaluation of antiviral and antitumor vaccine preparations aimed at cancer prevention.